1. Field of the Invention
The present invention relates to an optical disk of a phase change scheme and an optical disk apparatus which records data in the optical disk and reproduces recorded data.
2. Description of the Related Art
An optical disk apparatus which records data in an optical disk of a phase change scheme and reproduces recorded data has been put into practical use.
The optical disk is divided into a plurality of zones each having a plurality of tracks in the radial direction. The tracks in a zone have the same number of sectors, and the zones are different in the number of sectors per track. When data is to be recorded in the optical disk or recorded data is to be reproduced, the optical disk is rotated at a constant speed, and data is recorded or recorded data is reproduced by means of an optical head on the basis of clock signals having different frequencies depending on zones. This optical head is moved in the radial direction of the optical disk, and then data is recorded or recorded data is reproduced in a predetermined track.
One sector has a capacity of 725 bytes and is made up of a 52-byte pre-formatted area (which corresponds to a header) and a 673-byte recording area (user data recording area) (see U.S. Pat. No. 4,937,804).
In order to increase the number of times of recording repetition by randomly distributing mark recording positions obtained by non-crystallizing a recording film at laser spots (i.e., to increase the number of times of overwriting with respect to the recording film), the above optical disk of the phase change scheme employs a scheme of delaying recorded data in one way which is randomly selected from the 0 to 16 channel bits in units of channel bits by an optical disk apparatus. This scheme is called "a random shift" in recording.
Since the recording area is randomly shifted backward within a maximum of 16 channel bits (which correspond to 1 byte in 2-7 modulation), deterioration in optical disk is reduced in appearance.
Since the recording area is randomly shifted backward by a maximum of 16 channel bits (which correspond to 1 byte in the 2-7 modulation), sector format allows a gap section GAP before the recording area and a buffer section subsequent to the recording area to have a 1-byte variable length.
In the conventional optical disk of the phase change scheme, however, as a means for synchronizing bytes in reproduction (i.e., the optical disk apparatus recognizes the boundary of reproduction data and an ordinal byte position in the sector format which corresponds to the boundary during reproduction), only three bytes are prepared as a synchronization byte section SYN for performing data sync (sync: synchronization) after the gap section in which a random shift is executed. For this reason, if the synchronization byte section SYN cannot be detected due to a defect, dust, a scratch on the optical disk, and the like, bytes cannot be resynchronized in correspondence with the random shift, resulting in a failure in correct reproduction of data in the recording area.
That is, if the synchronization code section SYN as data sync before the data area in the recording area of the sector cannot be detected, bytes cannot be synchronized, and the data in the data area cannot be reproduced.